97

Sediment Transport and Channel Changes in an Aggrading Stream in the 

Puget Lowland, Washington

Mary Ann Madej

ABSTRACT

Recent land-use changes increased sediment yield in a fourth-order stream in western Washington from 22 t/km2 per year to 

185 t/km2 per year. In response, channel width increased and depth decreased, but channel gradient and mean flow velocity 

remained about constant. As a result of channel changes, sediment transport rates increased from 500 t/year to 4200 t/year. 

Active sediment constitutes 5 percent of stored sediment, and much sediment will be flushed from the system in about 20 to 

40 years.

Mary Ann Madej, Geologist, National Park Service, Redwood National Park, Arcata, California 95521.
98

INTRODUCTION

The sediment load of a stream is determined by such characteristics of a drainage basin as lithology, vegetation, precipitation, and land use, and it enters the stream system by 

a variety of erosional processes. If stability is,to be maintained, an equilibrium must exist between sediment entering 

the stream and sediment transported through the channel. 

Changes in sediment load can upset this balance and result 

in physical and biological changes in the stream system. 

The purpose of this study is to determine the source and 

volume of sediment introduced into a stream, and its subsequent distribution, rate of movement, and influence on channel geometry. Three approaches were used. First, a sediment 

budget quantifies the amount of sediment introduced into 

the stream channel. Second, sediment-transport equations 

predict how much sediment moves under conditions of 

various channel morphologies. Third, survey measurements 

indicate the volume and distribution of the deposited sediment.

FIELD AREA

Big Beef Creek, a gravel-bedded stream, was selected for 

study because high rates of sedimentation and channel 

instability during the last 10 years were evident and baseline 

data were available from records kept at the University of 

Washington Fisheries Research Station at the mouth of the 

creek. This fourth-order stream drains an area of 38 km2

on the Kitsap Peninsula in Puget Sound, western Washington (fig. 1). More than 85 percent of the drainage basin 
99

is forested with second-growth western red cedar (Thuja 

plicata Donn ex D. Don), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and associated species. The climate is 

characterized by cool, dry summers and mild, wet winters. 

Annual precipitation (predominantly rain) ranges from 1300 

to 1700 mm, and most major storms occur between December and March.

The dominant surficial geologic unit in the drainage basin 

dates from the Vashon (late Wisconsin) glaciation. A mantle 

of compact till, composed of a bluish-gray mixture of 

cobbles, pebbles, and silt and clay, covers much of the area. 

In addition, poorly sorted recessional outwash sands and 

gravels form many of the surface deposits in the Big Beef 

Creek drainage basin.

The upper 15 km2 of the Big Beef Creek basin has a low 

channel gradient (0.2 percent), an average hillslope gradient of 5 percent, and drains into an artificial lake. Because 

the lake acts as a sediment trap for suspended and bedload 

particles from the upper basin and most runoff and sediment discharge are generated below the lake, this study only 

considers a sediment budget for the lower drainage basin, 

where average channel gradient is 1 percent and hillslope 

gradients range from 10 to 70 percent. Drainage density below the lake is 2.5 km/km2

, as measured on 1:12,000 aerial 

photographs.

In 1969, the U.S. Geological Survey established a gaging 

station 1.6 km above the mouth of Big Beef Creek. The frequency of annual peak discharges was compiled from daily 

discharge records. The 10- and 50-year floods are about 30 

and 56 m3/second, respectively. No flood has exceeded 30 

m3/second in the past 10 years, and regional records indicate 

that no 50-year floods have occurred in the last 25 years. 

Thus recent changes in the Big Beef Creek basin occurred 

under fairly normal climatic patterns.

The principal activities in the drainage basin since the 

1850’s have been logging and some cattle grazing. Most virgin timber was logged by 1930 and a resurgence of logging 

activity occurred in the mid-1950’s, when harvest of secondgrowth timber was initiated. Urban development is becoming widespread and this decreases the amount of forested 

land as well. In 1942, 3 percent of the land was deforested, 

by 1965 the figure was 12 percent, and today it is almost 20 

percent. Likewise, road density tripled, from 1.2 km/km2 in 

1942 to 3.5 km/km2 at present.

Personnel at the University of Washington Fishery Research 

Station noted heavy sediment deposition at the mouth of 

Big Beef Creek during the winters of 1966-67 and 1967-68; 

in the summer of 1969, the lower 500 m of the creek were 

channelized artificially. In 1970, a weir 2 m high was built 

across the mouth of the stream, which has acted as a small 

dam and has caused deposition of bed material for several 

hundred meters upstream of the weir.

METHODS

Cross-Section Surveys

Fifty-five cross sections along the main channel of Big Beef 

Creek were surveyed periodically from 1969-78 by Cederholm (1972) and me (fig. 1). Gradients of channel thalweg 

and water surface also were determined in several locations.

Detailed records of channel changes at one cross section 

were obtained from U.S. Geological Survey gaging station 

records for 1969-79. These records (Forms 9-207 and 9-275) 

consist of bimonthly measurements of width, cross-sectional 

area, mean velocity, discharge, and stage relative to gage 

datum.

Regional Channel Geometry Survey

Because the channel geometry of Big Beef Creek was not 

determined until after the drainage basin had been disturbed 

by road construction and timber harvest, predisturbance 

geometry was predicted from a regional channel-geometry 

relation developed from relatively undisturbed drainage basins in similar geologic and geographic settings. Estimates 

of regional channel geometry were obtained by measuring 

bankfull width and depth, channel gradient, and sedimentsize distribution and correlating them with drainage area at 

20 sites on nearby streams (Madej 1978). Comparison of dimensions of the present Big Beef Creek channel with those 

from undisturbed channels draining the same area indicates 

the extent of changes in Big Beef Creek.

Sediment Studies

Size distribution of sediment in the main channel and 

tributaries was characterized with the pebble-count method 

described by Wolman (1954). Samples of channel gravel, 

and hilislope and bank material were also collected, sieved, 

and weighed.

Sediment distribution within the main valley of the drainage 

basin was determined by mapping the location and volume 

of gravel bars, overbank sand deposits, terraces, tributary 

deltas, and debris fans. Aerial photographs at a 1:12,000 

scale from seven flights between 1942 and 1976 allowed me 

to compare the locations and sizes of large gravel bars, log 

jams, and braided reaches through time. Area and location 

of mass movements were also mapped from photographs. 

Field checking provided more accurate estimates of volumes 

of material, as well as ages of mass-movement features 

dated by vegetation.

The size of the largest particle of bed material moved from 

a site by high flows during 1977 was obtained by observing marked rocks. Over 2,000 painted rocks were scattered 

on the upstream end of six gravel bars in the main channel 

and tributaries. The painted rocks ranged from 16 to 180 

mm (D50 = 64 mm). After high flows, the size of each rock 

found and the distance it had moved were recorded.
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Bedload-transport rates were estimated by the methods 

proposed by Einstein (1950), Meyer-Peter and Muller (1948), 

and Emmett (1976). Dr: H. W. Shen of Colorado State 

University supplied a computer program of the Einstein Bed 

Load Function. This program was used to compute bedmaterial load under various conditions representative of the 

changing channel geometry from 1969 to 1978.

RESULTS

Measurements

Table 1 lists the results of cross-sectional surveys of six 

reaches during three periods. Generally, aggradation occurred in lower reaches, degradation in middle reaches, and 

little change occurred in upper reaches, as illustrated in 

selected cross sections (fig. 2).

Net aggradation occurred in Reach 1 above the weir. Between 1969 and 1976, the bed aggraded 0.5-1 m in the lower 

300 m. Overbank flow deposited material over an area 70 m 

wide and built small levees to form new banks about 0.5 m 

above old banks. Alders rapidly revegetated overbank deposits and helped stabilize new banks. Overbank deposition 

is active, as indicated by several centimeters of sand burying 

young alders.

Net aggradation occurred in Reach 2 (fig. 2). The channel responded by assuming a more braided pattern in this 

reach and forming midchannel gravel bars. Bankfull width 

increased about 25 percent from 1973 to 1978, and bankfull 

depth decreased by a similar amount. Bankfull velocity 

(from U.S. Geological Survey records) remained about constant throughout this period.

Figure 2—Selected vertically exaggerated cross-sectional surveys 

in the main channel of Big Beef Creek.
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Bank erosion and scour took place at several locations in 

Reaches 3 and 4, which may be a response to previous aggradation. Where roads are still contributing large volumes 

of sediment, aggradation and growth of gravel bars is apparent. In contrast, the channel just below the lake (Reach 

5), which is located in a relatively undisturbed area, showed 

little change.

Longitudinal surveys indicate that, in the lower 2 km, channel gradient decreased from 0.090 to 0.085 between 1970 

and 1977. Total channel length increased from 9785 to 10 

010 m. Sinuosity increased from 1.30 to 1.32, and overall 

channel gradient decreased slightly.

Patterns of channel length changes correlate approximately 

with trends of cross-sectional changes. Where little cross-sectional change occurred, channel length remained constant. In 

degrading reaches, channel length increased; in the aggrading 

lower reaches of Big Beef Creek, channel length decreased.

The predicted and actual values of width for lower Big Beef 

Creek differ considerably (fig. 3), although the difference is 

negligible for the upper stream below the lake. Values for 

bankfull depth, channel gradient, and mean particle size are 

within limits of scatter in the regional pattern.

After one winter, 53 percent of the painted rocks were 

found. Large particles (>128 mm) did not move as far as 

intermediate sizes (45-90 mm). Surprisingly, small particles 

( 45 mm) did not move as far as those in the intermediate 

size class. Many of these small particles were found lodged 

behind large ones and were protected from further movement. Mean distance moved by intermediate-sized particles 

(45-90 mm) was 80 m.

Distribution of sediment stored in the channel is listed in table 

2. In side channels, the volume of sediment above bedrock was 

measured. In the main channel, sediment was only measured to 

the depth of scour which was taken to be 0.4 m, the maximum 

depth of degradation in cross-sectional surveys.

Log jams are temporary sediment traps for bed material 

moving downstream. Logging activity may increase the 

number and magnitude of log jams in a stream channel, 

which in turn may increase the deposition of gravel and 

sand. In a relatively undisturbed reach, four log jams were 

noted, or 2.5 jams/km of channel. In reaches with recent 

logging, the concentration was 12 jams/km and these jams 

were larger than those in undisturbed reaches. Log jams are 

associated locally with braided channels as the stream bends 

around the jams.

Suspended sediment was sampled periodically at 0, 0.8, and 

1.5 km above the mouth. Using U. S. Geological Survey daily discharge data, I constructed three suspended-sediment 

rating curves (Madej 1978). Average suspended-sediment 

load near the mouth of Big Beef Creek is 2325 t/year (110 

t/km2 per year), assuming the basin above the lake does not 

contribute any suspended sediment to downstream reaches.

Figure 3—Results of regional channel-geometry survey for Puget 

Lowland Streams (present dimensions for Big Beef Creek at 1.5 

km are indicated by an asterisk).
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CALCULATIONS

Sediment Budget

The amount of sediment entering Big Beef Creek can be 

quantified by examining the contribution of processes supplying hillslope material to channels. Sediment input in an 

undisturbed area was considered to enter channels by, soil 

creep and hillslope failures. Rate of soil creep under edaphic 

and climatic conditions in the Big Beef Creek drainage basin was estimated from published creep rates for other areas 

(table 3). Three studies have been conducted in temperate 

areas receiving about the same annual precipitation as Big 

Beef Creek (1300-1500 mm), and the coarse soil textures in 

these study areas are similar to the Big Beef Creek basin. 

Average rate of creep for the top 15-25 cm of soil for all 

the studies lies within a narrow range, and a value of 1.5 

mm/year was chosen as a reasonable estimate of creep rate 

in the Big Beef Creek drainage. In addition, tree throw 

(Dietrich et al., this volume) contributes the creep equivalent 

of 1.5 mm/year on steeper, wetter hillslopes. The extent of 

tree throw in the Big Beef Creek basin is not as great, and I 

estimate 1.0 mm/year of soil movement by tree throw, or a 

total rate of 2.5 mm/year.

The most active layer of soil in terms of creep is the upper 

zone, which is influenced by such processes as root growth 

and decay, tree fall, and burrowing of animals. In the Big 

Beef Creek basin, the observed thickness of this active layer 

is about 0.5 m on hillslopes of 25-45 percent, and it has a 

density of about 1.7 t/m3

.

Total length of first- to fourth-order channels in the lower 

drainage basin is about 57 500 m. Because soil transported 

downhill by creep enters channels through shallow bank failures on both sides of a channel, the total amount of material introduced by creep is: 5.75 x 104 m x 2 x 0.5 m x 0.0025 m/year 

x 1.7 t/m3 = 245 t/year = 10.6 t/km2/year. This is a minimum 

estimate, assuming soil deeper than 0.5 m contributes little 

material in comparison to the more active surface layer.

Material discharged through soil creep is transported either 

as bedload or suspended load, depending on the size of 

material. Suspended-sediment samples collected in Big Beef 

Creek have a maximum particle sizes of 1 mm. Textural 

analyses of soil in the Big Beef drainage basin indicate that 

40 percent of the active layer is greater than 1 mm. Thus, 

147 t of material discharged into the channel through soil 

creep become suspended load, and 98 t is transported as 

bedload.

Some bedload may break down into particles small enough 

to be carried as suspended load. Tumbling experiments 

with rocks of similar lithology and size as material in Big 

Beef Creek (Kuenen 1956) show that 10 km of transport is 

insufficient to break down much material. Average rate of 

attrition is 0.1 percent/km of transport.

Large discrete hillslope failures are the second major contributor to a stream’s sediment load. Field measurements of 

depth of slides averaged 0.6 m. During the last 10 years, 880 

m2 of land was affected by landslides that were not associated with roads, logging, or residential lots. So, the rate of 

mass wasting for undisturbed parts of the basin is: 880 m2

x 0.6 m x 1.7 t/m3 10 years = 90 t/year. These slides, which 

occurred on slopes >15 percent or at the headcuts of firstorder channels, were representative of an area of 6 km2

 in 

the lower drainage basin, so the rate can be expressed as (90 

t/year)/(6 km2) or 15 t/km2 per year. This rate is lower than 

estimates from other drainage basins in wetter, steeper environments (Swanston 1969, Dietrich 1975, Pierson 1977).

Because human activity significantly affected the remainder 

of the lower drainage basin, the rate of naturally occurring 

slides for the entire lower basin must be extrapolated from 

the rate of landsliding in the undisturbed sections. An area 

of 6 km2 has hillslopes <15 percent, and probably does not 

contribute sediment through landsliding. The remaining 17 

km2 contributes 15 t/km2 per year, or 255 t of sediment per 

year. This sediment is characterized by the size distribution 

of the average soil column. Thus, 153 t/year is suspended 

load, and the remainder becomes bedload.

Consequently, in an undisturbed state, sediment input into 

Big Beef Creek was:

Soil creep - 245 t/year 147 t/year of suspended load

98 t/year of bedload

Mass movements - 255 t/year 153 t/year of suspended load

102 t/year of bedload

 Total load - 500 t /year 300 t/year of suspended load

 (22 t/long per year) 200 t/year of bedload

This value for total sediment yield lies at the lower end of 

the range of sediment yields reported from drainages wetter, steeper, or both--such as 28 t/km2

 per year in western 

Oregon (U. S. Environmental Protection Agency 1971), 73 

t/km2 per year in southwest Alberta (Hollingshead 1971), 

and 105 t/km2 per year in British Columbia (Slaymaker and 

McPherson 1977).
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The Big Beef Creek drainage basin is no longer an undisturbed system because of changes in land use. In areas 

undergoing logging and road construction, rates of occurrence of mass movement may increase (Anderson 1954, 

Fredriksen 1970). In addition, processes such as sheetwash 

and rainsplash erosion--normally insignificant in humid, 

well-vegetated areas--may transport material from roads to 

channels.

Annual sediment contribution by disturbancerelated landslides was quantified by mapping volumes of slides originating on roads and cut areas, and dividing volumes by the age 

of slides. These slides were smaller but more numerous than 

naturally occurring slides on valley sides. Between 1968 and 

1977, an average of 845 t/year entered the stream channel 

through debris slides induced by land-use changes.

Roads are an additional source of sediment. Sheetwash on 

roadbanks and roadbeds, erosion of roadside ditches and 

sidecast material, roadbank gullying, and dry ravel from 

roadbanks are all active on roads in the Big Beef Creek 

basin. The volume of material eroded from roadside ditches 

was estimated in the field. Alluvial fans are not found at outlets of culverts draining ditches, and the largest painted rock 

moved in roadside ditches had a diameter of 64 mm. Thus, 

most eroded material probably reaches the stream channel. 

Roadside ditches contribute 440 t/year to the stream system.

Sheetwash carries sediment from roadbanks and roadbeds 

to stream channels. Few studies have directly measured 

amounts of sediment originating from roads, but these studies indicate the order of magnitude of sediment eroded from 

unpaved roads: Hornbeck and Reinhart (1964) - 9900 t/km2

per year for fresh roads and 125 t/km2 for abandoned roads; 

Diseker and Richardson (1961) - 20 000 to 60 000 t/km2

per year; and Wolman and Schick (1967) - 20 000 to 50 000 

t/km2 per year. L. Reid and T. Dunne (University of Washington) are currently studying erosion on logging roads, and 

their results for moderately used and abandoned roads are of 

the same magnitude as those of Hornbeck and Reinhart. Of 

these estimates, the 9900 t/km2 of Hornbeck and Reinhart 

seems the most reasonable to apply to the Big Beef Creek 

drainage basin, because these authors studied tractor-yarding skid roads under slope and precipitation conditions 

similar to those in the Big Beef drainage basin.

Although soil loss in the above studies was measured under 

a variety of conditions, sediment yields obtained were 

always far above the sediment yield in undisturbed areas. To 

apply these values directly to the Big Beef Creek drainage 

basin is risky because of variation in local conditions. Nevertheless, these values are useful estimates of soil loss from 

roads in the Big Beef Creek drainage.

Road prisms in the Big Beef Creek basin average 8.m in 

width. In 1977, 30 of the 65 km of unpaved roads were actively used. Using Hornbeck and Reinhart’s values of sediment yield, I computed soil loss on roads to be: .008 km [(30 

km x 9900 t/km2 per year) + (35 km × 125 t/km2 per year)] 

or 2410 t/km2

 per year. By examining the size distribution of 

lag deposits of the road surface, I estimated that 80 percent 

of the eroded material (1930 t) was transported as suspended 

sediment. Scour of roadside ditches contributes an additional 440 t/year. Thus a total of 2850 t/year is eroded from 

roads, of which 685 t is bedload.

The total sediment load of the stream is the sum of the contributions of natural hillslope processes and erosion processes from disturbed areas (table 4). Although these estimates 

in the sediment budget are approximate, their concordance 

with other measurements indicates they are of the correct 

magnitude. The estimate of 3000 t of suspended sediment/

year compares favorably with the 2325 t/year obtained from 

suspended sediment rating curves. Estimated bedload input 

is 1200 t/year; the volume of sediment deposited in the lowest kilometer of Big Beef Creek between 1970 and 1977 was 

4700 m3, or 9000 t, an average load of 1400 t/year. Some of 

the 1400 t/year may represent sediment moving out of storage from upstream reaches, rather than bedload input from 

hillslope processes.

The computed sediment budgets suggest that total load of 

the stream increased from 500 t/year under undisturbed 

conditions to over 4200 t/year at present. Even if absolute 

values of sediment input are contested, the apparent contrast 

of sediment loads under undisturbed and disturbed conditions are large and significant. Sediment Transport

Once sediment enters a channel, it is transported downstream during high flows. Bedload transport represents work 

done by a stream and is a function of the boundary shear 

stress generated at any given moment and place. Boundary 

shear stress, τb, produced by a fluid is calculated as:

where u* is related to velocity a at distance z from the channel boundary by

where ρ = density of the fluid, u*

 = shear velocity, and 0.4 = 

von Karman’s constant. The depth at which velocity appar-
104

ently goes to zero as the channel bed is approached is the 

roughness length of the boundary, zo, and is dependent upon 

particle size. It can be estimated by D65/30, where D65 is 

the grain diameter that is greater than 65 percent of the bed 

material (Graf 1971).

Bedload transport rates for the gaging site at Big Beef Creek 

(fig. 1) before and after disturbance were calculated from 

Einstein (1950), Meyer-Peter and MUller (1948), and Emmett (1976) (table 5). I have discussed their equations and 

the applicability to Big Beef Creek (Madej 1978). Dimensions of the undisturbed channel were estimated from 

regional channel-geometry results; those of the disturbed 

channel were obtained from survey measurements. Measured frequencies of discharge for each year were used. The 

three equations predict that an average of between 600 and 

1400 t of bedload/year would be transported past the gaging 

site.

INTERPRETATION

Present channel morphology allows greater bedload transport than the undisturbed channel (table 5). Analysis of 

boundary shear stress in the channel leads to the same 

conclusion. An increase in stream-channel capacity must be 

caused by an increase in τb. Using U.S. Geological Survey 

velocity and discharge measurements, I compared magnitude and distribution of shear stresses at four sites near the 

gaging station at equivalent discharges over a 4-year period 

(Made] 1978). Figure 4 shows the results of this analysis at 

one site, where erosion occurred at the right bank and the 

channel was 2 m wider in 1975 than in 1971.

Several common characteristics were found in the four 

analyses. The channel became wider and shallower through 

time (also shown by periodic cross-sectional surveys). Large 

gravel bars have low shear stresses above them and high 

shear stresses around them, perhaps caused by flow accelerating around the barrier. Most important, high shear stresses 

are found in areas of bank erosion, and the high stresses can 

be maintained over a shallower cross section. Thus, the ability of the channel to transport sediment is increased under 

the new channel configuration.

Channel Changes: Possible Modifications

The geometry of alluvial rivers can adjust to changing 

conditions imposed upon them (Mackin 1948, Leopold and 

Maddock 1953). In Big Beef Creek, an increase in sediment 

load was accommodated by changes in several interrelated 

hydraulic variables. Grain size and lithology of sediment 

supplied to a stream are variables independent of the stream 

system, as is discharge entering a stream. Other factors are 

semidependent and can undergo some modification by a 

stream in response to changes, i.e., flow resistance, mode 

of sediment transport, sediment-particle size, and channel 

pattern. Some parameters are almost totally functions of 

processes of the stream itself and can be considered to be 

dependent: velocity, width, depth, and channel gradient.

In response to an increase in sediment load, the more 

dynamic factors in a stream change to accommodate the increase. Adjustments are limited by certain physical relationships, however. The first of these is the continuity equation:

where Q = discharge, w = width, a = mean depth, and u = 

mean velocity.

The Manning equation (4) defines the relationship between 

a and d:

where s = slope of the stream-energy line and n = turbulent-friction coefficient (roughness). In addition, sediment 

transport, Qs, is here assumed to be dependent upon stream 

power:

Figure 4—Distribution of boundary shear stress in main channel, 

1.6 km above the mouth of Big Beef Creek.
105

The manner in which the Big Beef Creek channel changed 

is summarized in a previous section. Theoretically, however, 

these changes do not represent the only possible changes, 

and it is important to explain why the channel has been 

modified in this manner. Because transport of sediment.represents work accomplished, the amount of sediment transport was used as the criterion for capacity for work among 

various channel configurations. The Einstein method was 

used to compare the transport capability of several channel 

configurations.

Several combinations of width, depth, and velocity (restricted by continuity and roughness relationships) were 

tested with the Einstein computer program (fig. 5). An 

increase in width/depth ratio indicates increased bedload 

sediment transport (Schunm 1968, Pickup 1976). This holds 

true if velocity is held constant (line A in fig. 5), implying a 

decreasing roughness coefficient.

In a second set of calculations, velocity changed. If slope 

and roughness are considered to be a constant, k, the value 

of velocity from the Manning equation is kd2/3, and discharge is defined as: Q = kwd5/3. In this case (line B in fig. 

5), rates of sediment transport decrease with increasing 

width/depth ratio. Therefore a change in the roughness ‘n’ 

apparently is needed to increase transport rates.

Discharge measurements indicate that bankfull velocity 

remained about constant over the last 8 years, although the 

width-to-depth ratio, w/d, increased from 19 to 27. Line A in 

figure 5 suggests that w/d can increase indefinitely to transport increasingly larger sediment loads; however, in reality, 

stream width does not increase indefinitely. To understand 

what mechanism limits the increase and where these limits 

may be expected to occur, the effect of bed-particle roughness on flow must be considered.

Coarse surficial particles on a river-channel bed cause 

turbulence and energy loss in water flowing over them. The 

effect of these particles is most pronounced at low flows. 

Leopold et al. (1964, p. 160) described this phenomenon in 

gravel-bedded streams using a ratio of depth of water (d) 

to particle size (D84) of the armor layer. The relationship is 

described as:

where np is roughness from bed particles and g is the acceleration from gravity, 980 cm/second2

.

Hence roughness from bed particles is dependent only on 

depth of water and not on width or velocity. Values of n 

predicted by this method are minimum values, because 

other forms of roughness such as gravel bars, log jams, bank 

roughness, and meanders increase n. But water cannot flow 

over a rough bed with values of n below that predicted by 

the d/D84 relationship (equation 6).

At any given discharge, if width and depth are defined, 

velocity is known through the continuity equation. If velocity and slope remain constant as depth changes, roughness 

as defined by the Manning equation must change. Figure 6 

shows the relationships of width, depth, velocity, and roughness at bankfull discharge (10 m3

/second) in Big Beef Creek. 

Because no streams in the regional survey show a w/d less 

than 18, only those greater than 18 are considered. Actual 

ratios are below 27, but hypothetical cases with higher 

ratios are considered as well. Because survey measurements 

show little change in channel gradient, gradient is assumed 

constant in this analysis. Manning’s definition represents 

total roughness, nt, contributed by several factors; the value 

of nt decreases with increasing width and is given for each 

location considered.

With decreasing depth, roughness from bed particles, np, increases (from equation 6), and horizontal contours of figure 

6 indicate the value of np at different depths of flow.

Figure 6—Velocity and roughness constraints on the Big Beef 

Creek channel configuration at bankfull discharge (relationships 

determined from equations 3, 4, and 6).
106

Records from the U.S. Geological Survey gaging station 

show that mean velocity at bankfull stage remained between 

150 and 160 cm/second over an 8-year period. Only a small 

portion of the diagram represents channel configurations 

where bankfull discharge is accommodated within this 

velocity range (the stippled area of fig. 6). Because velocity remained constant despite changes in channel form, I 

assumed velocity would remain constant if the width/depth 

ratio increased, and so the stippled area also includes hypothetical configurations.

As long as discharge can flow with a roughness term greater 

than that predicted by the d/D84 relationship, the channel 

configuration is possible. Conversely, once bed-particle 

roughness is greater than the roughness needed to keep velocity constant, satisfying conditions of continuity, Q = wdu, 

becomes impossible. Thus, a hydraulic constraint prevents 

unlimited increases in channel width.

This relationship implies that the amount of sediment that 

can be transported as bedload is limited. Sediment transport can increase with width-to-depth ratio only as long as 

roughness-velocity constraints are met. If all sediment supplied to the channel cannot be transported at the maximum 

possible width/depth configuration, aggradation occurs.

Configurations that are not within the constant velocity 

envelope probably do not exist, because the amount of work 

necessary to create those situations is large. For example, 

point A in figure 4 is associated with a velocity of 213 cm/

second, and nt = 0.0195. Such a low value for total channel 

roughness is rarely, if ever, found in gravel-bedded streams 

(Barnes 1967). Alternately, point B in figure 4 represents 

a situation where velocity is 76 cm/second and nt = 0.087. 

Because np is only 0.008 at this depth, the nt value of 0.087 

would be difficult to reach even with the contribution of 

other causes of roughness, and this configuration is improbable.

Big Beef Creek does not display the maximum theoretical 

width of 30 m. Several reasons may account for this. A stable channel must have boundaries that can withstand shear 

stresses imposed by the flow. Using Graf’s method (1971), 

critical shear stress on channel walls in Big Beef Creek is 

between 55 and 150 dy/cm2, above which erosion occurs. 

These stresses are approached and probably exceeded at 

high flows (fig. 4); however, the resistance of banks caused 

by vegetation must be considered. A cover of Bermuda grass 

can resist stresses up to 480 dy/cm2 (Leopold et al. 1964). 

The thick mat of tree and grass roots on the banks of Big 

Beef Creek probably inhibits bank erosion.

Undercutting of banks below the root zone does occur, 

but not rapidly. The channel at the U.S. Geological Survey 

gaging station underwent the largest changes in 1973-74, 

when discharges were in excess of 20 m3

/second. Maximum 

discharge since that time has been 13 m3 second. More widening may occur when another high discharge is reached, 

but it seems unlikely under present vegetation and discharge 

conditions that the channel will erode 15 m to reach 30 m.

The valley widens near the mouth of Big Beef Creek, and 

channel width does reach 30 m, but this partly reflects weir 

and tidal action effects on channel geometry.

Storage of Sediment in the Channel

In several surveyed as well as unsurveyed reaches, much 

sediment eroded from roads and recently harvested hillslopes is obviously stored in the channel in the form of 

gravel bars. Field mapping indicates that the total volume 

of stored channel material is about 520 000 m3 (1 092 000 

t), of which 23 400 m3 (49 200 t) is active material (i.e., in 

unvegetated bars down to a scour depth of 0.4 m) (table 2). 

Most active material lies in the lower 6.5 km, where channel 

braiding and log jams are prevalent.

An estimate of potential residence time for active material 

under present conditions is obtained by dividing the amount 

by the bedload-transport rate after disturbance, 1000 t/year, 

or 50 years. An average rate of movement can be estimated 

by dividing channel length, 10.3 km, by 50 years, or 206 m/

year. Milhous (1975) obtained a particletransport rate of 234 

m/year through bedload measurements in an undisturbed, 

steeper channel with coarser gravel.

Relation of Channel Changes to Land Use

Changes in the Big Beef Creek channel can be related to 

land use in the drainage basin. Intense logging in the mid1950’s 2000 m above the mouth dramatically increased 

sediment load in that reach, and aggradation occurred. 

Assuming bed material moves 200 m/year, effects of excess 

sediment would be felt near the mouth in (2000 m)/(200 m 

per year), or 10 years, about 1965. In fact, local residents 

noted excessive sedimentation in the winters of 1966-68.

Present cross-sectional surveys show degradation in the 

logged reach and aggradation near the mouth, suggesting the 

slug of sediment is being transported downstream. A high 

bedload rate in this reach is also indicated by field observations. Channel length decreased, and the number and size of 

midchannel bars increased.

A similar trend is seen 6.2 km upstream (fig. 2C). Road construction and logging in the late 1960’s caused aggradation 

of the adjacent stream channel. Presently, net degradation is 

occurring there, and channel widening and braiding occur 

downstream for 0.8 km. This accumulation constitutes a 

second distinct wave of sediment in Big Beef Creek, and it 

will affect the stream for several years to come.
107

CONCLUSION

In forested drainage basins, sediment is supplied to stream 

channels by soil creep and mass movements. Channel form 

is adjusted to the amount of sediment supplied. Logging, 

road construction, and urban development remove vegetation, disrupt the ground surface, and cause accelerated 

erosion from sheetwash, mass movements, and rainsplash 

erosion, and hence increase the sediment load. Under undisturbed conditions, Big Beef Creek received 500 t of sediment per year. Land-use changes increased the load to 4200 

t/year. Construction of a weir at the mouth of the stream 

caused the bedload fraction to be caught above the weir, 

where an average of 1400 t of sand and gravel are deposited 

annually.

Stream-channel dimensions changed in response to increased sediment load. The channel in 1978 was wider and 

shallower than the 1970 channel, more gravel bars were 

present, and sinuosity decreased slightly in areas of high 

sediment transport. Mean flow velocity remained about 

constant. The changes result in an increase in shear stress 

along bed and banks, which in turn results in a higher rate 

of sediment transport. Before disturbance, sediment-transport rate of bedload was probably 300-600 t/year; at present, 

it is 600-1400 t/year.

Channel adaptations are restricted by hydraulic constraints 

described in the continuity and Manning equations. Velocity 

and slope are relatively conservative parameters, and most 

change is accommodated by width and depth.

The stream channel presently has about 1 000 000 t of 

sediment in storage. Active sediment, 49 000 t, moves an 

average of 200 m/year in the main channel. Thus sediment 

placed in the channel by present disturbances will take on 

the order of 20 to 40 years to be removed.

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